Alternate drivers should be chosen based on how well they work in a 4 cubic foot sealed enclosure (believe it or not). Choose low FS as Dan said, and look for the best efficiency. You can download a speaker program from somewhere and model different woofers in a 2nd order (sealed) box to find the best match.

Steve

ADDED NOTE: The driver does not know it's in a horn. The impedance plot will reflect a true sealed box. Think of the horn as a parallel resonator. The horn does not effect the driver, as was the point behind it's design. This way, unlike the original Imperial, sound from other speakers in the room will not activate the horn and create unwanted bass.

I found a great driver for my old Imps, but it fits Steves requirements well. It is the JBL136 a or h, or the pro version JBL 2231 a or h. The fs is 16hz, the qts is .2, the sens. is 97, and the recommended enclosure is 3 to 5 cu. ft. I am getting useable signal down to 16hz and strong signal at 20hz. It is fairly flat all the way up to 700hz were I cross over to my Edgarhorns.